Plastics and many other materials can be identified by their infrared (IR) reflectance or transmission spectrum. Each type--nylon, polyethylene, etc.--has its own IR characteristic spectrum. If a generally constant-intensity IR beam incident on a plastic is scanned through a range of wavelengths, and the intensity of the reflected or transmitted light is measured as a function of the wavelength, then the measured spectrum can be used to identify the type of plastic.
In addition, mixtures of plastics or other materials can be quantitatively analyzed. The reflectance or transmission spectrum of a sample can show that it is, for example, 50% nylon and 50% polyethylene. The proportion of octane in a sample of gasoline can be measured, or the amount of fat in chocolate bar.
Several types of IR spectrometers are known. Some use diffraction grating or FTIR technology; these are bulky, delicate, and slow. They are not suited to rapid identification of plastics, use in various locations such as in the field, or for handheld use.
Another known type uses an acousto-optical tunable filter (AOTF) such as that disclosed in U.S. Pat. No. 5,120,961 to Levin et al, U.S. Pat. No. 4,883,963 to Kemeny et al, and U.S. Pat. No. 4,052,121 to Chang, the entire contents of which patents are fully incorporated herein by reference. The acousto-optic tunable filter (AOTF) is based on a birefringent crystal, such as a crystal of TeO.sub.2 (tellurium dioxide) which acts as an electronically tunable narrowband filter, in which diffraction results from an acoustic pressure wave in the crystal.
If an acoustic wave traverses the crystal, the compression or pressure inside the crystal varies as the wave passes, causing a periodic variation in the refractive index. As crystal compression varies, so does the birefringence of a beam of unpolarized visible light or IR that passes through the crystal in a direction normal to its entry and exit faces. When sound having a certain acoustic wavelength is present in the crystal, the crystal acts as an optical filter passing that light or infrared having a wavelength proportional to the acoustic wavelength. Because the birefringent crystal acts as a frequency-selective narrowband optical filter, and sound of any acoustic wavelength can be passed through the crystal, any desired visible or IR wavelength can be selected at will, just by varying the frequency of an acoustic driver.
The acoustic driver is a second crystal of the piezo-electric type (quartz or lithium niobate, LiNbo), which is an acoustic transducer. Such a piezo crystal changes its size when subjected to an RF field.
Birefringent TeO.sub.2 bonded to piezo-electric LiNo, in which the LiNo is subjected to a sinusoidally-varying AC voltage applied across the face parallel to the birefringent crystal, will act as a swept-frequency optical filter. When the AC voltage impressed across the piezo crystal is at high radio-frequencies (RF) of 20-100 MHz, the acoustic wavelength corresponds to infrared (IR) light wavelengths. (One MHz is one million cycles per second.) The impressed voltage may be obtained from digital synthesizer, controlled by a software algorithm which determines the frequencies generated, and which can sequentially scan or hop in a random access fashion.
Broad-spectrum white light (from a halogen lamp, for example) which shines through the crystal (parallel to the junction between the birefringent and piezo crystals) will emerge as a beam having one optical frequency corresponding to the acoustic frequency of sound in the piezo crystal. Typical IR wavelengths selected by the AOTF filter are from 1-3 .mu.m (near infrared) or from 2-5 .mu.m (mid-infrared).
The tuned infrared beam can then be either reflected from, or transmitted through, a sample to determine the spectrum and identification of the sample. To identify the sample of plastic or other material, the swept-frequency beam of light is made to shine onto a surface of the undetermined material, which will reflect different proportions of the light falling onto it at each of the various frequencies. A photodetector can be used to pick up the reflected light and turn it into an electrical signal. Electronic circuits can then plot the pattern of the material's reflectance of IR or light frequency, and use that pattern to identify the material by matching the pattern with known patterns corresponding to various materials.
IR spectrometers can measure the proportion of a compound in a sample, by calibrating the circuitry to recognize samples having various percentages of compounds. The percentage can also be calculated according to Beer's law.
Compared to other spectrometer instruments such as diffraction gratings and the FTIR, the AOTF spectrometer has the advantages of no moving parts, high speed wavelength tuning, and small size. However, previous AOTF spectrometers have consisted of a fairly bulky and heavy electronics and optical modules, so that the instrument cannot be portable and handheld or low cost. In addition, for measurement of remote samples in-situ, it has been necessary to use fiber optics to pass the light between the instrument and the sample.
In these conventional AOTF systems the IR coming out of the AOTF is passed into an optical fiber or bundle of fibers, which traps the light and passes it along the bundle, but with substantial loss, especially at longer IR frequencies. (Optical fibers can pass visible light with little attenuation, but IR is strongly absorbed.) The end of the fiber or bundle can then be placed near the surface of the material, and reflected light picked up by other fibers of the bundle to be conveyed back to a photo-detector.
While the optical-fiber arrangement allows the light-emitting end to be placed easily at any point, it has several drawbacks in addition to the aforementioned inefficiency in transporting IR. First, optical fibers are expensive. Second, they are fragile. Third, the coupling between a light source, such as the lamp/AOTF, and the fiber is very inefficient. Only a small fraction of the available light can be conveyed into and along the fibers. Furthermore, because optical fibers are quite small the light they convey scatters widely from their ends, and is dispersed, so still more light is lost even if the end of the fiber bundle is placed almost against the surface of the material under test.
The high losses of optical fibers require a very bright lamp in front of the AOTF, of 50 to 100 W. These high wattage lamps typically require cooling fans. The high-wattage lamps and fans in turn require a larger power supply, and the larger power supply may require heat dissipating fin plates or fans of its own, which then draw still more power and increase the size and bulk still further. The frame or housing must be larger and heavier to support the additional parts. Expense is increased and portability decreased.
Prior-art AOTF spectrometers are locked into a "catch-22" size and weight restriction. The bulk of the lamp/AOTF units prevents them from being held up to a sample, so fiber optics are used to convey the IR from the housing to the sample; but since fibers waste energy, the housing must be large and heavy. Previous workers in the field have not achieved a portable AOTF spectrometer because they have not realized the root of the size/weight problem.
Another, related drawback of conventional AOTF spectrometers is that RF power is delivered to the piezo crystal via a coaxial cable from a power amplifier located in a separate housing. This arrangement wastes electrical energy, both by attenuation in the cable and also because of losses due to impedance-matching the RF amplifier to the cable, and then the cable to the crystal. (The cable typically has an impedance of around 50 ohms, but the piezo crystal impedance is much lower and varies with frequency.) Moreover, previous AOTF spectrometers have applied more RF power to the AOTF crystal than was needed.
Since the electrical power drain from too much RF power and from using both fiber optics and coaxial cables is too great for battery power, the power supply must include circuitry to transform 120 volt AC, and a power line cord and plug must be provided. The unit then is still more difficult to transport and use.
Those of ordinary skill in the spectrometer art have been unable to achieve handheld portability by removing these power drains. Previous workers in the field apparently did not realize that the energy drains resulting from separating the components were the key problem keeping AOTF spectrometers from being portable. They placed the detector and the RF amplifier in separate housings requiring them to be connected by coaxial cable causing bulkiness and power loss, and the AOTF crystal and lamp in another housing, further hindering portability. Furthermore, they did not understand that the RF power could be reduced, allowing the RF amplifier to be placed into the same enclosure or housing as the crystal.